Safety fuse for high-voltage transformers



Jan. 16, 1951 H. KOTT SAFETY FUSE FOR .HIGH-VOLTAGE' TRANSFORMERS Filed Dec.

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Hcrma BY Patented Jan. 16, 1951 SAFETY FUSE FOR HIGH-VOLTAGE TRANSFORMERS Hermann Kott, West Orange, N. J. Application December a1, 1946, Serial No. 719,563

3 Claims. (Cl. 200-1135) This invention relates to electrical fuse de- Y vices for the protection of electrical circuits and electrical devices and has for its object the provision of an electrical fuse operative to protect circuits carrying high voltages.

Another object is to provide an electrical fuse adapted to protect the secondary winding of high voltage transformers against the deleterious effect of voltages exceeding the safety limit of the insulation employed in the transformer.

Still another object is to provide a safety fuse for high voltage transformers limiting the voltage present in the secondary winding thereof to a determined maximum voltage.

A further. object is to provide a safety fuse means in an electrical circuit including a high voltage transformer limiting the voltage in the secondary circuit of the transformer to a safe maximum voltage.

Other objects will be apparent as the invention is more fully hereinafter disclosed.

In accordance with these objects I have discovered that the voltage in an electrical circuit including a high voltage transformer may be closely controlled or limited to a safe maximum voltage by the provision of a fuse element which is electrically connected in the primary circuit of the said transformer to carry the current load of said primary circuit which fuse element also is arranged to function as one electrode in a gaseous conduction discharge device, the opposite electrode of which is electrically connected to carry the voltage of the secondary winding of the said transformer with the space gap between said electrodes being selected to provide a breakdown voltage therebetween approximating the desired maximum voltage in the said secondary and the said fuse element being designed to be destroyed rapidly by the arc discharge between the electrodes thereby to open the primary circuit of the transformer and interrupt the current flow in the said primary of the transformer.

The invention is adapted to wide modification without essential departure therefrom as one skilled in the art will recognize from the specific embod ments thereof hereinafter given.

As one specific embodiment of the invention, but not as a limitation thereof, I will disclose the invention as it has been adapted to the control and regulation of the voltages in the secondary winding of high voltage transformers employed in circuits including high voltage electrical discharge devices of the gaseous conduction type, such as those known in the art as fluorescent lamps.

In the operation of fluorescent lamps a voltage of 10,000 to 12,000 volts normally is required to initiate the discharge in the lamp. After the discharge is initiated the operating voltage drops rapidly to a voltage of 6,000 to 8,000 volts. The transformer windings are designed generally to withstand for relatively short time intervals surges of voltage as high as 15,000 volts but usually the insulation will puncture at 10,000 to 12,000 volts within a few minutes. Thus it will 1 be seen that in the event of lamp failure, and in the absence of a protective fuse such as that provided by the present invention, transformer failure is likely to occur whenever lamp failure occurs or whenever a lamp is removed from the circuit.

Before further disclosure of the present invention reference should be made to the accompanying drawings, wherein:

Fig. 1 is a circuit diagram of a typical electrical circuit including a high voltage transformer and a fluorescent lamp, said circuit diagram also including the fuse means of the present invention;

Fig. 2 illustrates a modified form of the fuse means shown in Fig. l;

Fig. 3 is a sectional view of the fuse means of Fig. 2 taken along plane 3-4;

Fig. 4 is a view illustrating the fuse means of Fig. 2 in its blown condition;

Fig. 5 illustrates a second modified form of the fuse means of the present invention; and

Fig. 6 illustrates a third modified form of the fuse means of the present invention.

Referring to Fig. 1, the electrical circuit shown includes a high voltage transformer T having primary and secondary windings P and S, respectively, with the secondary winding 8 having a grounded mid-tapG and the opposite high voltage ends thereof electrically connected by conductors cd to the opposite electrodes of fluorescent lamp L. The primary winding P is shown electrically connected in series with the fuse means F of the present invention, conductors a--b being the feed lines connecting to a source of low voltage alternating current, such as a house lighting circuit.

Fuse means F of the present invention consists essentially of an envelope ll enclosing a pair of spaced apart conductors Ill-l0, the ends of conductors l0|0 passing through the en velope II in a seal M excluding the atmosphere from the envelope interior.

Conductors l0l0 consist of metal of high electrical conductivity, or low resistance, with recognize.

sufficient spring strength to permit the ends thereof to be pressed towards each other against the Spring strength of the conductors a suillcient distance to bring contacts 202I mounted on opposite conductors III-I I into electrically conducting engagement.

' The conductors III-II are held in this position by means of fuse element I2 whose opposite ends are spot welded to the ends of conductors I Fuse element I2 consists of low thermal expansion electrically conductive metal and, preferably, consists of a small diameter wire or thin sectined strip of said metal. The particular size wire or thickness of strip employed for fuse element I2 is selected to provide a determined time interval of resistance of the fuse element to buming through in an arc discharge under the tension applied by contacts IO-II. A large number of low thermal expansion alloys are known in the art and are adapted for use as a fuse element in the present invention. Typical of such alloys is that iron-nickel alloy known under the tradename Invar." The particular composition of fuseelement I2 forms no part of the present invention.

Destruction of fuse element I2 is effected by means of an arc discharge drawn between the fuse element I2 and spaced electrode I2 passing through cap It closing the open end of envelope II, said electrode I I being electrically connected by conductors I to to one side of the secondary s of transformer T. The spacing between the ,end of electrode I3 and the fuse element I2 is selected with respect to the atmosphere and pressure of the atmosphere within envelope I I to give a determined break-down voltage therebetween, said breakdown voltage being the desired limit ing voltage on transformer T.

The atmosphere within envelope II may be widely varied, as may the pressure of the atmosphere. without essential departure from the invention, as one skilled in the art will readily Air or any of the gases, hydrogen, nitrogen, carbon dioxide. and one of the monatomic noble gases may be employed, if desired, the breakdown voltages of these gases at various pressures being well known to those skilled in the art.

For most purposes, however, I have found that air at atmospheric pressures is a satisfactory atmosphere to employ within envelope II as well as being the most economically practical atmosphere. With air at atmos heric pressures a s ace gap of about 5 mm. provides a breakdown voltage of about 3.500 volts between electrode II and fuse element I2. At this voltage a determined time interval is required to initiate the arc discharge between the electrodes I2 and I2 which time interval decreases as the voltage increases becoming substantially zero at voltages above about 10,000 volts. At 5,000 to 6.000 volts, however, the time interval for an arc discharge to initiate between electrode I3 and fuse element I2 is still longer than the time interval for the initiation of an arc discharge in lamp L. Therefore, upon energization of primary P a surge of high voltage in secondary S of 10,000 to 12,000 volts for the short time interval required to initiate the discharge in lamyp L is insufllcient to initiate an arc discharge in fuse element F, as one-half of this voltage only is impressed on electrode I3. However, if for any reason the lamp L fails to therein destroyed with resultant springing apart of conductors III-III and opening of contacts 2II-2I and consequent breaking of the primary circuit and deenergization of transformer T.

It is; of course, necessary to have conductors IO-III spring apart, upon the burning through of fuse I2, suiliciently to separate contacts 20-2I a distance preventing an arc discharge therebetween. This distance varies with respect to the current at any given voltage in the primary circuit. Most installations carry about 10 to 20am- Pe 'es in the primary circuit and with this amperage at 110 volts a spacing between contacts 2l-2I of about 2 mm. is adequate to extinguish an arc therebetween.

Referring now to the specific embodiment of Figs. 2, 8 and 4, it is advantageous to provide means to repair or replace fuse element or wire I2 after the wire I2 has been burned through or "blown. This is provided for by comprising the envelope I I of two telescoping parts, Ho and Ilb, together with meanssuch as extension shoulders 22-22 on part Ila to limit the extent to which part I I b may be telescoped over part I is thereby to control or regulate the space gap between electrode I! carried by part Ilb from fuse wire I2 carried by part Ila. As the ends of conductors Il-Ifl extend beyond the end of part IIa replacement of fuse wire I2 is relatively simple as is also re-assembly of the two parts lid and Nb to a fixed spacing between electrode II and fuse wire I2.

It is also possible, by varying the diameter of fuse wire I2 to vary the time interval required, after an arc discharge initiates between electrode I3 and fuse wire I2, to burn through the wire. Ordinarily, under the conditions above given, a wire of the preferred Invar metal having a diameter of 5 to 6 mils will burn through in about 1 6th second after the discharge initiates. This time interval appears satisfactory for most installations to prevent damage to the transformer.

Larger or smaller diameter wire will Provide.

longer or shorter time intervals.

Referring next to Fig. 4, the relative position of conductors I0-III after the fuse wire I2 has been burned through by the arc discharge is shown. The size, shape and configuration of contacts 20-2I may vary widely without departure from the invention. The essential function of the contacts is to carry the maior portion of the current load in the primary circuit so that variations in the diameter or thickness of fuse wire I2 maybe made without effecting the functioning of the device.

Referring next to Fig. 5, the modification shown is one contemplated for use in primary circuits carrying a small current load within the current carrying capacity of fuse wire I2. In this modification, while metal of the'same composition heretofore disclosed for use in fuse wire I2, may be employed, low melting point fuse wire may be substituted therefor without departure from the invention, if desired.

In the modification of' Fig. 5, which is adapted to be manufactured by standard lamp manufacignite or the secondary circuit is opened within a short time interval thereafter the arc discharge turing practices, substantially any desired gas at any desired pressure may be substituted for the air atmosphere of the previous described examples, with accompanying advantages, as one skilled in the art will recognize. The provision of a standard screw-type lamp base II on the device or the bayonet type base I Ia (of Fig. 6) is optional. In this type of device a plurality of standard fuses each having a determined breakdown voltage within a wide range of voltages may be produced to provide for a variety of service demands or conditions. Ordinarily, for example, in fluorescent lamp installations a voltage range of 3,000 to 20,000 volts is an ample range to cover. A fuse device for each 1,000 volts within this range may be provided through the selection of the proper combination of atmosphere, pressure and electrode spacing in the device of Fig. 5.

Referring now to the modification of Fig. 6, an adjustable electrode I3 is shown. Envelope ll may be replaced by the telescoping envelope Ila-I lb of Fig. 3, if desired, without departure from the invention. The advantage of an adjustable electrode 13 is to provide means to preset the space gap of the device to any desired break-down voltage. A suitable scale mounted on the interior surface of envelope Il may be utilized to facilitate spacing to determined distances.

From the above disclosure and specific examples given it is believed that one skilled in the art may readily duplicate the invention and that many modifications and departures from the specific examples given will be apparent to those skilled in the art, and all such modifications and departures are contemplated as may fall within the scope of the following claims:

What I claim is:

1. A protective device for high voltage transformers, said device comprising a fuse having a base provided with a pair of terminals, a pair of spring arm continuations of said terminals extending above the said base in substantial parallelism, each said spring arms having an electrical contact located thereon in a position to engage each other when the said spring arms are moved towards each other, a fuse wire securing the upper ends of said spring arms together in contact closing position in opposition to the spring arm tension, a housing enclosing the spring arms and base from the atmosphere and leaving the terminals exposed, and an electrode extending through the housing with its inner end in determined spaced relation to the said fuse wire to provide a desired break down voltage therebetween and with its outer end'providedwith an external terminal.

2. The protective device of claim 1, wherein said electrode is adjustable toward and away from the said fuse wire.-

3. The protective device of claim 1, wherein said housing is tubular and is comprised of two telescoping sections, one carrying the said electrode and the other the said base and spring arms.

HERMANN KOTT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 819,608 Sherwood May 1,-1906 1,677,692 Stokes July 17, 1928 1,848,152 Bieger Mar. 8, 1932 1,859,377 Boudion May 24, 1932 1,977,191 Levenstein Oct. 16, 1934 2,099,500 Sanders Nov. 16, 1937 2,146,498 Bradley Feb. 7, 1939 2,463,812 Seaman Mar. 8, 1949 FOREIGN PATENTS Number Country Date 472,312 Great Britain Sept. 22, 1937 

